Current regulation flowing through windings of a continuous current electric motor may be carried out in a PWM mode by sensing the total current entering in or exiting from the motor. The motor may be, for example, a spindle motor for hard disk applications. A block diagram of a typical half-bridge drive stage of a spindle motor is depicted in FIG. 1. Each half-bridge comprises a low-side transistor referenced to ground and a high-side transistor connected to a supply voltage Vmotor.
Current sensing may be performed also through a sense resistor Rs, as shown in FIG. 2. The sense resistor Rs is connected between the common connection point of the source terminals of the low-side MOS and ground, as shown in the figure, or the line at the supply voltage Vmotor (not depicted). In this case, during the current decay interval, with the low-side MOS transistors M2, M4, M6 turned on, no current flows throughout the sense resistor Rs. This results in a loss of information about the intensity of the motor current. In other applications, a sense FET (not depicted) is used instead of the sense resistor Rs.
The overall current absorbed by the motor is compared with a reference REF. Each time the motor current surpasses this reference, the current limiter comparator (COMP) switches and the half-bridge drive stage is configured such that the motor current circulates in a current decay electrical path. This has a slow decay recirculation of the motor current for a constant and programmable time (K_Toff). For the whole duration of the constant decay interval K_Toff, the drive stage is configured with the three low-side MOS transistors on.
As an alternative, it is possible to obtain a slow decay recirculation of the current by turning on all 3 high-side MOS transistors M1, M3, M5. Typically, a digital filter may also be used (from 100 ns up to 0.5 μs, for example) for masking possible spurious switching of the comparator COMP that may give false information.
At the end of the current decay interval, the output stage is configured to energize again the motor windings (Ton). This allows the motor current to attain again the threshold REF, and the loop is repeated.
The described regulation loop is characterized by having a limited and non-instantaneous response time (minimum Ton). When the motor current attains the reference threshold, there is a response delay of the regulation loop before starting the slow recirculation phase, and thus making the current decrease below the regulation threshold REF.
FIG. 3 depicts a graph of the regulated current that flows throughout the motor as a function of the regulation threshold REF. The continuous line represents a desired (or ideal) behavior, and the dashed line represents a current behavior.
For high currents the ideal and current behavior are practically the same. For small values of the regulation threshold REF, the problems tied to the loop delay (that makes driving intervals have at least a minimum duration Ton) do not allow the real current to track the desired value.
In particular applications, characterized for example by the use of a motor having a low phase impedance and supplied with a high voltage, the loop delay may cause a loss of current as depicted in FIG. 4. It may happen that, because of the minimum driving (energization) interval Ton due to the delay of the regulation loop, at each alternating cycle between driving (energization) intervals Ton and the slow decay intervals Toff of constant duration, the average value of the current increases progressively. Thus, the current flowing throughout the motor differs in an increasing manner from the desired regulation value REF.
Moreover, while driving a spindle motor, the phase of the back electromotive force that normally opposes the current flow may vary. It may vary such that during the decay interval there is an increase instead of a decrease of the current.
A regulation of the motor current to address or at least limit eventual regulation losses would be desirable. The eventual regulation losses may be due to the driving interval having a minimum duration, such as due to the characteristics of the control loop, for example.